The present invention relates to a polyethylene naphthalate film suitable for the base of magnetic recording medium which is superior in heat resistance, mechanical properties, film flatness, and handling qualities. More particularly, the present invention relates to a biaxially oriented polyethylene naphthalate film produced by the electrostatic cooling method from polyethylene naphthalate having a resistivity of 1.times.10.sup.7 to 5.times.10.sup.8 .OMEGA..cm as measured in the molten state, said film having a center line average roughness Ra (.mu.m) and a film thickness T (.mu.m) which satisfy the following formula. ##EQU2##
One of the major films in industrial use at present is polyethylene terephthalate film. The polyethylene terephthalate film is utilized to the widespread use because of its outstanding thermal property, mechanical property, chemical resistance, and weather resistance.
Ordinarily, the magnetic recording medium is produced by forming a magnetic layer on the surface of the base film according to a coating method or a vacuum evaporation method.
The magnetic recording medium is used as a base material of a magnetic tape or magnetic disc of an audio apparatus, a video and a computer memory. In the case of forming the magnetic layer on the base film by the coating method, a dispersion containing magnetic particles composed of .gamma.-iron oxide, .gamma.-iron oxide coated with cobalt, chromium dioxide and iron, a binder resin, a dispersing agent, an abrasives, an electrification preventing agent, a lubricant, a hardening agent, etc. is coated on the surface of the base film and after being subjected to magnetic orientation treatment, the thus obtained magnetic tape is dried.
Also, in the case of the vacuum evaporation method, a magnetic thin layer of cobalt-nickel alloy is formed by using a continuous vacuum evaporation machine.
Polyethylene terephthalate film is now indispensable for the base material of magnetic recording medium on account of its high strength, high Young's modulus, and excellent dimensional stability as compared with other films.
Recently, the magnetic recording medium is being improved very rapidly and the requirements for the base film has been made more vigorous. In the field of video recording, for example, equipments are being miniaturized and lightened weight. This movement requires the development of an 8-mm wide base film in addition to the conventional 0.5-inch wide base film.
The new base film for magnetic recording medium is required to thin and to increase mechanical strength. However, such requirements cannot be met with the conventional biaxially oriented polyethylene terephthalate film produced from polyethylene terephthalate by the sequential biaxial orientation in the longitudinal and transverse directions. The polyethylene terephthalate film has usually a Young's modulus of 400 to 600 kg/mm.sup.2 which is lower than the desired level. Film obtained by the so-called re-orientation method has an improved Young's modulus of 600 to 900 kg/mm.sup.2 and also improved mechanical properties, but it is so poor in dimensional stability at high temperatures and accordingly the obtained film is of no practical use.
Recently, biaxially oriented film of polyethylene naphthalate has become generally known. The biaxially oriented polyethylene naphthalate film is superior in Young's modulus and dimensional stability at high temperatures to polyethylene terephthalate film. Using polyethylene naphthalate film as the base film for magnetic recording medium is described in, for example, Japanese Patent Publication Nos. 48-29541 (1973) and 56-19012 (1981) and Japanese Patent Laying-Open (KOKAI) No. 50-45877 (1975).
It is generally known that polyethylene naphthalate film has good mechanical properties and dimensional stability at high temperatures. It is difficult to put the polyethylene naphthalate film to practical use, because it is poor in film flatness and handling qualities, especially in the case of thin one, if it is produced by the known process. It is apparent that polyethylene naphthalate film will be of great industrial value if these disadvantages are overcome.
With the foregoing in mind, as a result of the present inventors' extensive studies on polyethylene naphthalate film, it has been found that polyethylene naphthalate film is highly improved in film flatness and handling qualities if it is produced by the electrostatic cooling method from polyethylene naphthalate having a resistivity as measured in the molten state in a certain range and if the surface roughness of the film and the thickness of the film satisfy a certain relational formula, and based on the finding the present invention has been attained.